Make sure that resource_label_event.epic_id exists

resource_label_event.epic_id was added as part of EE version of
db/migrate/20180726172057_create_resource_label_events.rb, but if
user used CE version in that time and then upgraded to EE, then this
column was missing.

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180920043317) do
+ActiveRecord::Schema.define(version: 20180924070647) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

ee/changelogs/unreleased-ee/label-event-migration-ee.yml

+---
+title: Make sure that epic_id column exists in resource_label_events when upgrading
+  from CE to EE.
+merge_request:
+author:
+type: fixed

ee/db/migrate/20180924070647_add_label_event_epic_column.rb

+# frozen_string_literal: true
+
+class AddLabelEventEpicColumn < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    # When moving from CE to EE, this column may already exist
+    return if column_exists?(:resource_label_events, :epic_id)
+
+    add_reference :resource_label_events, :epic, null: true, index: true, foreign_key: { on_delete: :cascade }
+  end
+
+  def down
+    # epic_id is deleted in db/migrate/20180726172057_create_resource_label_events.rb
+  end
+end